Add base safety for grid override template mutation

Author: BrianMichell

src/mdio/builder/templates/base.py

@@ -92,9 +92,8 @@ def build_dataset(
         self._builder = MDIODatasetBuilder(name=name, attributes=attributes)
         self._add_dimensions()
         self._add_coordinates()
-        # Ensure any coordinates declared on the template but not added by subclass overrides
-        # are materialized with generic defaults. This keeps templates override-agnostic while
-        # allowing runtime-augmented coordinate lists to be respected.
+        # Ensure any coordinates declared on the template but not added by _add_coordinates
+        # are materialized with generic defaults. This handles coordinates added by grid overrides.
         for coord_name in self.coordinate_names:
             try:
                 self._builder.add_coordinate(
@@ -104,7 +103,7 @@ def build_dataset(
                     compressor=compressors.Blosc(cname=compressors.BloscCname.zstd),
                     metadata=CoordinateMetadata(units_v1=self.get_unit_by_key(coord_name)),
                 )
-            except ValueError as exc:  # coordinate may already exist from subclass override
+            except ValueError as exc:  # coordinate may already exist
                 if "same name twice" not in str(exc):
                     raise
         self._add_variables()
@@ -253,14 +252,21 @@ def _add_coordinates(self) -> None:
             )
 
         # Add non-dimension coordinates
+        # Note: coordinate_names may be modified at runtime by grid overrides,
+        # so we need to handle dynamic additions gracefully
         for name in self.coordinate_names:
-            self._builder.add_coordinate(
-                name=name,
-                dimensions=self.spatial_dimension_names,
-                data_type=ScalarType.FLOAT64,
-                compressor=compressors.Blosc(cname=compressors.BloscCname.zstd),
-                metadata=CoordinateMetadata(units_v1=self.get_unit_by_key(name)),
-            )
+            try:
+                self._builder.add_coordinate(
+                    name=name,
+                    dimensions=self.spatial_dimension_names,
+                    data_type=ScalarType.FLOAT64,
+                    compressor=compressors.Blosc(cname=compressors.BloscCname.zstd),
+                    metadata=CoordinateMetadata(units_v1=self.get_unit_by_key(name)),
+                )
+            except ValueError as exc:
+                # Coordinate may already exist from subclass override
+                if "same name twice" not in str(exc):
+                    raise
 
     def _add_trace_mask(self) -> None:
         """Add trace mask variables."""